What's the point of an interactive Ruby subshell?

Question

Starting up an interactive Ruby shell in the Terminal ('irb'), one can continue to open up irb subshells endlessly. What's the point of this?

How about the need to try something without contamination of previously defined variables or classes, without opening a new terminal window and starting IRB there? — the Tin Man, May 03 '13 at 20:15
@theTinMan actually, classes continue to be defined in the irb subshell (just like modules). when defined in the subshell, they are avaiable even after closing the subshell. — tessi, May 03 '13 at 21:00

score 9 · Accepted Answer · edited May 23 '17 at 12:04

So far I've seen three usefull things irb subsessions can do for you:

undefine local variables
change self of an irb session
irb is a part of a great set of tools

undefine local variables

The nested irb starts a new subsession in which all local variables (not classes, modules etc.) are not defined any more.

irb(main):001:0> a = 1
#=> 1
irb(main):002:0> irb
irb#1(main):001:0> a
  NameError: undefined local variable or method `a' for main:Object from (irb#1):1

change self for an irb session

irb(main):001:0> self
#=> main
irb(main):002:0> irb "Hello World"
irb#1(Hello World):001:0> self
#=> "Hello World"
irb#1(Hello World):002:0> length
#=> 11

Note: This is also known as "change binding" of an irb session.

By the way: It's possible to change the binding without opening a subsession (cb, irb_change-binding both do that for you). But it's more convenient to get back to the old binding with subsession.

The best thing is, that irb is just one of a useful set of commands

irb: start a new subsession
jobs: list subsessions
fg: switch to a subsession
kill: kill a subsession

See this insteresting SO answer for details.

What's the point of an interactive Ruby subshell?

1 Answers1